Disinfection, Residuals, and Disinfection Byproducts
Inactivation versus byproduct formation
Disinfection credit is expressed as CT: disinfectant concentration in mg/L times contact time in minutes, corrected by a baffling factor that reflects actual flow hydraulics. Each disinfectant has its own CT table by organism, pH, and temperature. Free chlorine is effective against Giardia but poor against Cryptosporidium; UV and ozone provide Crypto credit, and UV dose is measured in mJ/cm2 rather than CT. Chloramines are weaker but far more persistent in the distribution system.
Chlorine demand is the chlorine consumed by reduced substances before a free residual appears. With ammonia present, the breakpoint curve shows chloramine formation, then destruction, then free chlorine beyond roughly a 7.6 to 1 weight ratio of chlorine to ammonia nitrogen. Deliberate chloramination typically targets about 3 to 5 parts chlorine per part ammonia nitrogen to favor monochloramine and limit dichloramine taste.
DBP control links precursor removal to finished water quality: TOC removal by enhanced coagulation, bromide control, pH adjustment, and reduced water age. TTHM and HAA5 are regulated as locational running annual averages of 80 and 60 ug/L; ozone can form bromate and chlorine dioxide forms chlorite. Chloraminated systems must monitor for nitrification - falling total chlorine, rising free ammonia, and dropping pH in the distribution system.
Traps: CT questions often give total contact time and expect the baffling factor reduction; temperature corrections lower required CT in cold water for chlorine; and chloramine residual reported as total chlorine hides nitrification.
Worked example
Residual 1.0 mg/L free chlorine after 30 minutes in a basin with a 0.5 baffling factor: effective CT = 1.0 x 30 x 0.5 = 15 mg-min/L. Compare that to the required CT for the target organism and temperature before claiming credit.